a capacitive-type acceleration sensor has heretofore been used in a vehicle. In the capacitive-type acceleration sensor, a change in the capacitance of a sensor element caused by a change in the acceleration is converted into an electric signal through a capacitance-voltage (C-V) conversion circuit, and the converted electric signal is differentially amplified through a differential amplifier circuit that includes a sample-holding circuit and a switched capacitor circuit thereby to produce a sensor output (e.g., U.S. Pat. No. 6,483,322 corresponding to JP 2002-40047A).
The capacitive-type acceleration sensors are generally constructed as represented by sensor circuit diagrams of FIGS. 2 and 3. In those sensors, a substrate a forming a sensor element 10 is separated from a substrate B forming a detector circuit 20. The detector circuit 20 includes a C-V conversion circuit 21, a differential amplifier circuit 22 and a control signal generation circuit 23 that produces control signals to various parts. The detector circuit 20 applies a voltage to moving electrodes 1a, 1b or to fixed electrodes 2a, 2b constituting the sensor element 10. The circuit 20 further processes signals by receiving differential signals from the sensor element 10. Concerning the substrates a and B which are separate from each other, the potential of the substrate a is fixed to a DC voltage (sensor drive voltage Vdd (FIG. 2) or GND potential (FIG. 3)), while the potential of the substrate B is fixed to the sensor drive voltage Vdd (e.g., U.S. Pat. No. 6,240,782 corresponding to JP 11-295336A).
To obtain a sensor output signal maintaining a high sensitivity by using the thus constituted a capacitive-type acceleration sensor, it becomes necessary to use the C-V conversion circuit 21 and the differential amplifier circuit 22 of high multiplying factors. However, the potential of the substrate A forming the sensor element 10 is fixed to the DC voltage. If this potential is applied to the C-V conversion circuit 21, the frequency noise is superposed on the differential signals input to the input terminals of the C-V conversion circuit 21 due to mismatching of impedances of the inverting input terminal and non-inverting input terminal of the C-V conversion circuit 21 with respect to the power source. As a result, the frequency noise is amplified at the time of C-V conversion and is added to the sensor output signals. This makes it difficult to obtain sensor characteristics of a high degree.